Very low current oscillator with variable duty cycle

ABSTRACT

A low current oscillator circuit comprising a comparator for driving an output signal. A first capacitor chain is coupled to the comparator. The first capacitor chain is configured for setting a first input voltage of the comparator. A second capacitor chain is also coupled to the comparator. The second capacitor chain is configured for setting a second input voltage of the comparator, wherein the first capacitor chain and the second capacitor chain determine a first voltage level and a second voltage level of oscillation of the comparator. The first capacitor chain and the second capacitor chain are free of DC current flow.

TECHNICAL FIELD

The present invention relates to the field of low-power integratedcircuits. More particularly, the present invention relates to lowcurrent oscillator circuits.

BACKGROUND ART

Minimizing power consumption is critical in extending operating time inportable or battery power devices. The lower the power consumption ofthe circuit elements of, for example, a handheld cell phone, the longerthe handheld cell phone can provide its functionality to the user.Accordingly, circuit elements of a given device are designed for minimalpower consumption.

Low power consumption constraints generally apply to all elements of anintegrated circuit device, such as, for example, processors, outputdrivers, displays, etc. The circuit elements are designed to interactand function with each other, and must adequately perform their requiredfunction while remaining within the desired power consumptionconstraints. Oscillator circuits comprise one of the most fundamentalcircuit elements of electronic devices.

Oscillators are used to produce a wide variety of periodic signals(e.g., clock signals, etc.). Many of the components of an integratedcircuit device rely on clock signals generated by oscillator circuits toproduce their functionality. For example, modern analog to digitalconverters (ADCs) function by using low jitter external clock source.ADCs are specified to operate with a clock signal having a duty cyclethat varies across a specified range.

Similarly, most signal sampling mechanisms common to data communicationapplications rely on high-quality clock signals to sample incoming data.It is becoming increasingly common for such sampling mechanisms tofunction on both the rising edge and the falling edge of a clock signal,thereby making duty cycle control an important factor. As is well known,a clock signal's duty cycle refers to a ratio of one clock phase widthto the entire clock period.

A problem exists, however, with prior art oscillator circuits. Asdescribed above, high-quality clock signals are critical to functioningof many different functional blocks of an integrated circuit device. Toproduce such high-quality clock signals, and to provide a preciseadjustable duty cycle, prior art oscillator circuits include a largenumber of transistors, current sources, and other circuit elements.These elements tend to increase power consumption, thereby reducingbattery life of a handheld device. Additionally, a large number ofcircuit elements increase the amount of silicon area dedicated to theoscillator circuit, thereby increasing the cost of the overall device.

Prior art FIG. 1 shows a diagram of a typical prior art oscillatorcircuit 100. The prior art oscillator circuit 100 includes twoamplifiers 151-152. The oscillation is driven primarily from the actionof the amplifiers 151-152, which function as comparators. The output ofthe comparators 151-152 are respectively coupled to the NAND gates121-122 as shown. The NAND gates 121-122 are coupled in a feedback loopas depicted, which oscillates between a high-value and a low value. Theoutput of the oscillator circuit 100 is derived from the output of theinverter 123.

The oscillator circuit 100 includes the current sources 101-105 asshown. The high voltage level and the low voltage level between whichthe oscillator circuit 100 oscillates is set by the voltages V₁ and V₂as derived from the current from the current source 101 flowing througha resistor 111 and a resistor 112. The voltages V₁ and V₂ are coupled tothe positive input of comparator 151 and the negative input ofcomparator 152 as shown. In this manner, the current source 101 (e.g.,I₁) creates the voltages V₁ and V₂.

The current source 102 and the current source 103 bias the comparators151-152 as shown. The current source 104 and the current source 105 areused to charge and discharge the capacitor C₁, and thereby produce theoutput signal at node 130. As shown in prior art FIG. 1, the currentsource 104 charges the capacitor C₁ to the voltage V₂ and the currentsource 105 discharges the capacitor C₁ to the voltage V₁.

One problem with prior art oscillator circuit 100 is that it consumestoo much power. As shown in prior art FIG. 1, the oscillator circuit 100requires five current sources 101-105 in order to function. Each ofthese current sources represents a power consuming element. Anotherproblem is the fact that the prior art oscillator circuit 100 requirestwo comparators 151-152, which both consume power.

Yet another problem with the prior art oscillator circuit 100 is thefact that it requires a constant current flow, and thus a constant powerdrain, from the current source 101 across the resistor 111 and theresistor 112. These current flows are required in order to createvoltages V₁ and V₂.

Thus, what is required is a solution for implementing a very low powerconsumption oscillator circuit for an integrated circuit device. Therequired solution should provide an adjustable duty cycle that can beset in accordance with the needs of a given application. Additionally,the required solution should require fewer circuit elements incomparison to prior art oscillator circuit implementations.

SUMMARY OF THE INVENTION

Embodiments of the present invention provide a solution for implementinga very low power consumption oscillator circuit for an integratedcircuit device. Embodiments of the present invention provide anadjustable duty cycle that can be set in accordance with the needs of agiven application. Additionally, the embodiments of the presentinvention require fewer circuit elements in comparison to prior artoscillator circuit implementations.

In one embodiment, the present invention is implemented as a low currentoscillator circuit comprising a comparator for driving an output signal.A first capacitor chain is coupled to the comparator. The firstcapacitor chain is configured for setting a first input voltage of thecomparator. A second capacitor chain is also coupled to the comparator.The second capacitor chain is configured for setting a second inputvoltage of the comparator, wherein the first capacitor chain and thesecond capacitor chain determine a first voltage level and a secondvoltage level of oscillation of the comparator (e.g., the high level andthe low level of the voltage swings of the output of the comparator).The first capacitor chain and the second capacitor chain are both freeof DC current flow, thereby providing for a very low power consumptionof the oscillator circuit. In one embodiment, the ratios between thecapacitors of the capacitor chains determine the first and secondvoltage levels of oscillation. In one embodiment, a switch circuit iscoupled to the comparator to set the duty cycle of the oscillation ofthe comparator.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example and not by way oflimitation, in the figures of the accompanying drawings and in whichlike reference numerals refer to similar elements.

Prior art FIG. 1 shows a diagram of a typical prior art oscillatorcircuit.

FIG. 2 shows a diagram of a very low current oscillator circuit inaccordance with one embodiment of the present invention.

FIG. 3 shows a first graph and a second graph depicting signals of theoscillator circuit in accordance with one embodiment of the presentinvention.

FIG. 4 shows a graph of the overall power consumption of the oscillatorcircuit in accordance with one embodiment of the present invention.

FIG. 5 shows a schematic diagram of an oscillator circuit in accordancewith one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the embodiments of theinvention, examples of which are illustrated in the accompanyingdrawings. While the invention will be described in conjunction with thepreferred embodiments, it will be understood that they are not intendedto limit the invention to these embodiments. On the contrary, theinvention is intended to cover alternatives, modifications andequivalents, which may be included within the spirit and scope of theinvention as defined by the appended claims. Furthermore, in thefollowing detailed description of the present invention, numerousspecific details are set forth in order to provide a thoroughunderstanding of the present invention. However, it will be understoodby one of ordinary skill in the art that the present invention may bepracticed without these specific details. In other instances, well knownmethods, procedures, components, and circuits have not been described indetail as not to unnecessarily obscure aspects of the present invention.

Embodiments of the present invention provide a solution for implementinga very low power consumption oscillator circuit for an integratedcircuit device. Embodiments of the present invention provide anadjustable duty cycle that can be set in accordance with the needs of agiven application. Additionally, the embodiments of the presentinvention require fewer circuit elements in comparison to prior artoscillator circuit implementations. Embodiments of the present inventionand their benefits are further described below.

FIG. 2 shows a diagram of a very low current oscillator circuit 200 inaccordance with one embodiment of the present invention. As illustratedin FIG. 2, the oscillator circuit 200 includes a single comparator 251.The oscillator circuit 200 also includes a first current source 201 anda second current source 202.

The oscillator circuit 200 embodiment is configured as an ultra lowcurrent oscillator circuit. The comparator 251 is configured for drivingan output signal, which is taken at node 230. The oscillator circuit 200includes two capacitor chains for setting the voltage levels of theoscillation. The resistors R₁ and R₂ and the capacitors C₁ and C₂comprise the first capacitor chain. Similarly, the resistors R₃ and R₄and the capacitors C₃ and C₄ comprise the second capacitor chain, asshown.

In the present embodiment, the voltage levels of oscillation are set bythe two capacitor chains. As shown in FIG. 1, the comparator 251 hasthree inputs 252, 253 and 254. Two of these inputs (e.g., 253 and 254)are used to set the levels of the oscillation (e.g., the low voltagelevel of the oscillation and the high voltage level of the oscillation).In the present embodiment, the input 253 is coupled to V₁ of the firstcapacitor chain and the input 254 is coupled to V₂ of the secondcapacitor chain. The V₁ and V₂ voltages are created by the ratios of thecapacitors of the respective chains. For example, in the oscillatorcircuit 200 embodiment as shown, the V₁ voltage is determined by theexpression V₁ =C₁/(C₁+C₂) and the V₂ voltage is determined by theexpression V₂ =C₃/(C₃+C₄).

Thus, in contrast to prior art oscillator circuits where a current froma current source must flow across resistors in order to make the inputvoltages, the V₁ and V₂ voltages are created by the first and secondcapacitor chains that are completely free of DC current flow. In thismanner, the oscillator circuit 200 embodiment of the present inventioncompletely eliminates a current flow requirement for setting inputvoltages for the comparator 251. This aspect greatly reduces the currentflow within the oscillator circuit 200 embodiment, thereby greatlyreducing the power consumption.

Referring still to FIG. 2, the oscillator circuit 200 includes a switchcircuit comprising a capacitor C_(A) and a switch φ as shown. The switchcircuit functions by initiating the oscillation in the comparator 251. Acurrent source 201 is connected to the capacitor C_(A) and the switch φ.In the present embodiment, the switch circuit functions by setting theduty cycle of the oscillator circuit 200. The node 235 between thecurrent source 201 and a switch circuit is coupled to the input 252 ofthe comparator 251.

The current source 201 (e.g., I₁) functions by charging the capacitorC_(A) to the voltage V₂ (e.g., the high voltage level). The switch φfunctions by discharging the capacitor C_(A) to the voltage V₁ (e.g.,the low voltage level). The configuration of the switch φ and thecapacitor C_(A) determines the duty cycle of the output signal (e.g.,the signal from output node 230). This duty cycle can be adjusted byadjusting switch φ and the capacitor C_(A) (e.g., in accordance with therequirements of the overall device or system application). A DC currentpath to ground for the current source 201 exists only when the switch φis closed.

In the oscillator circuit 200 embodiment, the output of the comparator251 is coupled to a feedback circuit comprising the transistors 211-214as shown. The feedback circuit functions by driving the switching actionof the switch φ. An output of the oscillator circuit 200 is taken fromthe node 230.

Depending upon the overall system in which the oscillator 200 isincluded, the output taken from node 230 can be shaped by additionalexternal circuit elements (not shown). For example, external filteringelements can be coupled to the output in order to smooth the outputinto, for example, a sine wave. External filtering elements can becoupled to the output to shape the output into a sawtooth wave, or thelike. In this manner, the oscillator 200 can provide a wide variety ofdifferent periodic signals used in electronic devices.

It should be noted that the oscillator circuit 200 embodiment of thepresent invention includes only one comparator (e.g., comparator 251),in comparison to prior art oscillator circuit implementations whichincluded two or more comparators. The single comparator 251 onlyrequires a single current source 202 (e.g., I₂), as supposed to the twoor more current sources required in prior art oscillator circuitimplementations. This aspect further reduces power consumption of theoscillator circuit 200 in comparison to the prior art.

It should also be noted that the oscillator circuit 200 embodiment ofthe present invention has a low component count in comparison to priorart oscillator circuit implementations. For example, by reducing thenumber of included comparators (e.g., to one) and by reducing the numberof current sources, the component count, and thus the silicon arearequired to manufacture the oscillator circuit 200, is reduced. Thereduced silicon area results in a corresponding reduction in cost.

These aspects enable the oscillator circuit 200 embodiment of thepresent invention to implement a very low power consumption oscillatorcircuit for an integrated circuit device. The low power consumptionbenefits are provided while enabling duty cycle adjustment andoscillation voltage level adjustment. These features are provided whilealso reducing manufacturing costs in comparison to prior art.

FIG. 3 shows a graph 301 and a graph 302 depicting signals of theoscillator circuit 200 in accordance with one embodiment of the presentinvention. The horizontal axis of both the graphs 301 and 302 representstime. The vertical axis of the graphs 301 and 302 represents voltage, inthe scale shown. The graph 301 shows the voltage at the node 235 of theoscillator circuit 200, as the capacitor C_(A) charges over time fromthe current source 201 and is discharged by the switch φ. The graph 302shows the square wave output at node 230. Both graphs 301-302 are shownbeginning from an initial power on.

FIG. 4 shows a graph of the overall power consumption of the oscillatorcircuit 200 in accordance with one embodiment of the present invention.As shown in FIG. 4, during normal operation, the oscillator circuit 200draws a constant, very low amount of current. In this embodiment, theoscillator circuit 200 draws approximately 50.248 nano-Amps.

FIG. 5 shows a schematic diagram 500 of an oscillator circuit inaccordance with one embodiment of the present invention. The diagram 500of FIG. 5 shows a more detailed illustration of oscillator circuit 200of FIG. 2. The oscillator circuit of the diagram 500 functions insubstantially the same manner as the oscillator circuit 200 described inthe discussion FIG. 2 above.

The correspondence between the schematic diagram 500 and the oscillatorcircuit 200 of FIG. 2 is now described. In the diagram 500 illustration,the capacitor 501 corresponds to capacitor C_(A). The transistors 502correspond to the switch φ. The node 503 corresponds to the voltagelevel V₁ and the node 504 corresponds to the voltage level V₂. Thecapacitor 505 corresponds to the capacitor C₁. The resistor 506 and theresistor 507 correspond to the resistors R₁ and R₃. The capacitor 508and capacitor 509 correspond to capacitors C₃ and C₄. Capacitor 510corresponds to the capacitor C₂. The resistors 511 and 512 correspond tothe resistors R₂ and R₄. The transistors 513-516 correspond to thetransistors 211-214. The output 520 corresponds to the output node 230.

The foregoing descriptions of specific embodiments of the presentinvention have been presented for purposes of illustration anddescription. They are not intended to be exhaustive or to limit theinvention to the precise forms disclosed, and many modifications andvariations are possible in light of the above teaching. The embodimentswere chosen and described in order best to explain the principles of theinvention and its practical application, thereby to enable othersskilled in the art best to utilize the invention and various embodimentswith various modifications as are suited to the particular usecontemplated. It is intended that the scope of the invention be definedby the claims appended hereto and their equivalents.

1. A low current oscillator circuit, comprising: a comparator fordriving an output signal; a first capacitor chain coupled to thecomparator and for setting a first input voltage of the comparator; anda second capacitor chain coupled to the comparator and for setting asecond input voltage of the comparator, wherein the first capacitorchain and the second capacitor chain determine a first voltage level anda second voltage level of oscillation of the comparator free of DCcurrent flow.
 2. The low current oscillator circuit of claim 1, furthercomprising: a switch circuit coupled to the comparator and forinitiating an oscillation in the comparator.
 3. The low currentoscillator circuit of claim 2, further comprising: a feedback circuitcoupled to an output of the comparator, the feedback circuit alsocoupled to the switch circuit for driving the switch circuit.
 4. The lowcurrent oscillator circuit of claim 2, further comprising: a capacitorincluded in the switch circuit; a current source coupled to capacitorand for charging the capacitor to the high voltage; and a switch coupledto the capacitor for discharging the capacitor to the low voltage. 5.The low current oscillator circuit of claim 1, further comprising: afirst capacitor and a second capacitor included in the first capacitorchain; and a third capacitor and a fourth capacitor included in thesecond capacitor chain, wherein a node between the first capacitor andthe second capacitor determines the first voltage level of oscillationand a node between the third capacitor and the fourth capacitordetermines the second voltage level of oscillation.
 6. An integratedcircuit system including a low current oscillator circuit, comprising: acomparator for driving an output signal; a first capacitor chain coupledto the comparator and for setting a first input voltage of thecomparator; a second capacitor chain coupled to the comparator and forsetting a second input voltage of the comparator, wherein the firstcapacitor chain and the second capacitor chain determine a first voltagelevel and a second voltage level of oscillation of the comparator freeof DC current flow; and an output signal line coupled to an output ofthe comparator for providing an oscillation signal to external circuitelements of the integrated circuit system.
 7. The integrated circuitsystem of claim 6, further comprising: a switch circuit coupled to thecomparator and for initiating an oscillation in the comparator.
 8. Theintegrated circuit system of claim 7, further comprising: a feedbackcircuit coupled to an output of the comparator, the feedback circuitalso coupled to the switch circuit for driving the switch circuit. 9.The integrated circuit system of claim 7, further comprising: acapacitor included in the switch circuit; a current source coupled tocapacitor and for charging the capacitor to the high voltage; and aswitch coupled to the capacitor for discharging the capacitor to the lowvoltage.
 10. The integrated circuit system of claim 6, furthercomprising: a first capacitor and a second capacitor included in thefirst capacitor chain; and a third capacitor and a fourth capacitorincluded in the second capacitor chain, wherein a node between the firstcapacitor and the second capacitor determines the first voltage level ofoscillation and a node between the third capacitor and the fourthcapacitor determines the second voltage level of oscillation.
 11. A lowcurrent oscillator circuit, comprising: means for driving an outputsignal using a comparator; means for setting a first input voltage ofthe comparator using a first capacitor chain coupled to the comparator;and means for setting a second input voltage of the comparator using asecond capacitor chain coupled to the comparator, wherein the firstcapacitor chain and the second capacitor chain determine a first voltagelevel and a second voltage level of oscillation of the comparator freeof DC current flow.
 12. The low current oscillator circuit of claim 11,further comprising: means for initiating an oscillation in thecomparator using a switch circuit coupled to the comparator.
 13. The lowcurrent oscillator circuit of claim 12, further comprising: means fordriving the switch circuit using a feedback circuit coupled to an outputof the comparator, the feedback circuit also coupled to the switchcircuit.
 14. The low current oscillator circuit of claim 12, furthercomprising: a capacitor included in the switch circuit; means forcharging the capacitor to the high voltage using a current sourcecoupled to capacitor; and means for discharging the capacitor to the lowvoltage using a switch coupled to the capacitor.
 15. The low currentoscillator circuit of claim 11, further comprising: a first capacitorand a second capacitor included in the first capacitor chain; and athird capacitor and a fourth capacitor included in the second capacitorchain, wherein a node between the first capacitor and the secondcapacitor determines the first voltage level of oscillation and a nodebetween the third capacitor and the fourth capacitor determines thesecond voltage level of oscillation.
 16. The low current oscillatorcircuit of claim 15, wherein a first ratio of the first capacitor andthe second capacitor determines the first voltage level and a secondratio of the third capacitor and the fourth capacitor determines thesecond voltage level.
 17. The low current oscillator of claim 11,further comprising: means for determining a duty cycle of an oscillationin the comparator using a switch circuit coupled to the comparator.